Electronic converters for light sources including, for example, at least one LED (Light Emitting Diode) or other solid-state lighting devices, may supply a direct current output. Such a current may be steady or else may vary in time, for example, in order to set the brightness of the light emitted by the light source (so-called dimming function).
FIG. 1 shows a possible lighting system, including an electronic converter 10 and a lighting module 20 including, for example, at least one LED L.
For example, FIG. 2 shows an example of a lighting module 20 including, for example, a LED chain, i.e. a plurality of LEDs connected in series. For example, in FIG. 2 four LEDs L1, L2, L3 and L4 are shown.
Electronic converter 10 usually includes a control circuit 102 and a power circuit 12 (for example a switching supply AC/DC or DC/DC), which receives as input a supply signal (for example from the mains) and supplies as output, through a power output 106, a direct current. Such current may be steady or may vary in time. For example, control circuit 102 may set, via a reference channel Iref of power circuit 12, the current required by LED module 20.
For example, such a reference channel Iref may be used to regulate the intensity of the light emitted by lighting module 20. In fact, generally, a regulation of the light intensity emitted by LED module 20 may be achieved by regulating the average current flowing through the lighting module, for example by setting a lower reference current Iref or by switching off power circuit 12 through a Pulse Width Modulation (PWM) signal.
However, if module 20 is supplied with a regulated voltage, i.e. if converter 12 is a voltage generator, it is typically necessary to arrange a current regulator connected in series with light sources L, in order to limit the current. In this case, the dimming function may also be implemented via such a current regulator, for example:
a) either by selectively activating or deactivating such a current regulator through a driving signal, e.g. a PWM signal,
b) if an adjustable current regulator is used, by setting the reference current of such a current regulator.
Generally speaking, there are many types of electronic converters, which are divided mainly into isolated and non-isolated converters. For example, non-isolated converters are “buck”, “boost”, “buck-boost”, “Cuk”, “SEPIC” and “ZETA” converters, while isolated converters are “flyback”, “forward”, “Half-bridge” and “Full-bridge” converters. Such converter types are well known to the skilled in the art.
For example, FIG. 3 shows a circuit diagram of a ZETA converter which operates as a DC/DC converter. The person skilled in the art will appreciate that an input AC current may be converted into a direct current via a rectifier, for example a diode-bridge rectifier, and possibly a filtering capacitor.
Substantially, a ZETA converter includes an electronic switch S, a diode D, two inductors L0 and L1 and two capacitors C0 and C1, and therefore represents a fourth-grade converter.
In the presently considered example, converter 12 receives as input, via two input terminals 110, a voltage Vin and supplies as output, via two output terminals 106, a regulated voltage Vo or a regulated current io.
Specifically, the first terminal of input 110 is connected through switch S to a first terminal of inductor L1 and the second terminal of input 110 is connected directly to the second terminal of inductor L1 and represents ground GND.
In the considered example, the first terminal of inductor L1 is connected through capacitor C1 to the cathode of diode D, and the second terminal of inductor L1 is connected directly to the anode of diode D. The anode of diode D, i.e. the second terminal of inductor L1, is also directly connected to output 106, specifically to the second terminal of output 106.
The cathode of diode D is connected via the second inductor L0 to the first terminal of output 106. Finally, capacitor C0 is connected in parallel with output 106, i.e. directly to the terminals of output 106.
In the considered example, a load RL is connected to such an output 106, which for example may be the previously described lighting module 20.
As previously mentioned, the control may take place in current or in voltage. To this purpose a control unit 112 is typically used which drives switch S so that the output voltage Vo or the output current io is set to a desired value, such as for example the reference current Iref. To this purpose it is possible to use, in a manner known per se, a sensor adapted to detect current io or voltage Vo.
Referring to FIGS. 4A and 4B, a possible driving of such a ZETA converter will be described.
Specifically, as shown in FIG. 4A, during a first operation interval switch S is closed and diode D is OFF, i.e. diode D is inversely biased. In this case, inductor L1 saves the energy received from the input and capacitor C1 supplies energy, through output switch L0 and capacitor C0, to load RL. In this condition, the currents flowing through inductances L1 and L0 increase in a substantially linear way, while no current flows through diode D.
On the contrary, as shown in FIG. 4B, during a second operating interval switch S is open and diode D is ON. In fact, diode D is directly biased, because the bias of the voltage across inductance L1 is inverted. In this condition, the currents flowing through inductors L1 and L0 decrease in a substantially linear way. Specifically, the energy saved in inductance L1 is mainly transferred to capacitor C1 and the load receives energy mainly from inductor L0. Therefore, the current flowing through diode D is:iD=iC1+iLo  (1)
Details about the operation of such a ZETA converter are described for example in the paper by Huai W E I, et al., “Comparison of basic converter topologies for power factor correction”, IEEE Proceedings Southeastcon '98, p. 348-353, 24-26th Apr. 1998, Orlando, Fla., the content whereof is incorporated herein as a reference.
FIG. 5 shows an alternative embodiment of a ZETA converter, wherein the arrangement of capacitor C1 and of inductor L0 is different.
In the presently considered embodiment, the first terminal of inductor L1 is connected directly to output 106, specifically to the first terminal of output 106. On the contrary, the second terminal of inductor L1 is connected, through capacitor C1, to the anode of diode D, wherein the cathode of diode D is connected directly to the first terminal of output 106, i.e. to the first terminal of inductor L1.
In the considered embodiment, the anode of diode D is connected through inductor L0 to the second terminal of output 106.
Finally, in this case, too, capacitor C0 is connected in parallel to output 106, i.e. directly to the terminals of output 106.
Therefore, in the presently considered embodiment, the positions of capacitor C1 and of inductor L1 have changed: in FIG. 3 they were connected in series between the first terminal of inductor L1 and the first output terminal, while capacitor C1 and inductor L0 are connected in series between the second terminal of inductor L1 and the second output terminal in FIG. 5.
However, the general operating principle is substantially unaltered.
Typically, both operating intervals are repeated periodically with a fixed frequency, wherein the energy transfer is controlled via a PWM signal, i.e. the duration of the first and of the second interval are variable, while the sum of the durations is constant.
The skilled in the art will appreciate that such a PWM driving and the control of the durations of the operating intervals are well known and can be implemented, for example, through a feedback of the output voltage or current through an error amplifier. For example, in the case of a control by current, the duration of the first interval is increased until the (average) output current reaches a predetermined threshold.
In the known state of the art it has been moreover proposed to drive such a ZETA converter with a sort of driving called “soft switching”, wherein switch S is switched when the voltage across switch S is zero (zero voltage switching, ZVS). As a matter of fact, this sort of driving may reduce the switching losses and the electromagnetic interference (EMI).
For example, the paper by TSAI-FU, et al., “Design optimization for asymmetrical ZVS-PWM zeta converter”, IEEE Transactions on Aerospace and Electronic Systems, Vol. 39, Iss. 2, p. 521-532, April 2003, the content of which is incorporated herein as a reference, describes the use of an active clamp to such a purpose.
Specifically, as shown in FIG. 2 of said paper, a ZETA converter may be transformed into an isolated ZETA converter by replacing inductor L1 with a transformer T including a primary winding T1 and a secondary winding T2. Specifically, transformer T may be modelled as an ideal transformer with a given turn ratio 1:n, an inductance Lm connected in parallel with primary winding T1, representing the magnetization inductance of transformer T, and an inductance Lr connected in series with primary winding T1, modelling the leakage inductance. Moreover, switch S may also be arranged between the primary winding of the transformer and ground, which enables the use of an N-MOS transistor. Subsequently, an “active clamp” including an electronic switch (S2 in the paper) and a capacitor (capacitor Cc) is added to said isolated ZETA converter. According to the teaching of the above-mentioned paper, switches S1 and S2 are then driven with 4 driving modes, which are shown in FIGS. 3 and 4A-4B of said paper.
Essentially, the paper by WU TSAI-FU describes that such a converter is adapted to switch the switches S1 and S2 at zero voltage and the diode at zero current. Specifically, in section “F. Selection of Resonant Inductor and Clamping Capacitor” there is stated that the zero voltage switching may be obtained if leakage inductor Lr of the transformer has a stored energy which is sufficient to discharge the capacitance of switches and other capacitances (see equation (35) of the paper).
On the basis of such considerations, the paper indicates two relations (equations (37) and (39) of the paper) which enable to determine the minimum value for the leakage inductance of the transformer, as a function of the current flowing through switches S1 and S2, the input voltage, the duty cycle and the value of capacitance Cr.
Moreover, the paper proposes to dimension the clamping capacitor (Cc in the paper) as a function (see equation (41) of the paper) of the leakage inductance, of the duty cycle and of the switching frequency, so that a half-period of the resonance frequency of the leakage inductance Lr and of the clamping capacitor is higher than the maximum time during which switch S1 is open.
For example, in the article a leakage inductance is used which equals 20 μH and a clamping capacitor with 0.22 μF.
However, the inventor has observed that this kind of dimensioning of components has a number of drawbacks. In fact, the dimensioning of capacitor Cc causes the resonance current of leakage inductance Lr and of clamping capacitor Cc to be different from zero when switch S1 is opened or when switch S2 is closed. Therefore, in this case, the diode of the ZETA converter does not switch at zero current, which is visible for example in FIG. 14 of the paper. Moreover, as described at “C. Selection of power switches and diode”, switches must have a low capacitance, because otherwise the zero voltage switching of switches might be lost, or the leakage inductance of the transformer should be very high. Therefore, a typical value for the capacitance of switches is 42 pF.
Summary
According to various embodiments, an electronic half-bridge ZETA converter is provided. Various embodiments also concern a related method for operating an electronic half-bridge ZETA converter, and a corresponding method for designing an electronic half-bridge ZETA converter.
As previously mentioned, the present description concerns an electronic half-bridge ZETA converter supplying a power signal through an output.
In various embodiments, the electronic half-bridge ZETA converter includes a transformer with a primary winding and a secondary winding, wherein each winding includes at least a first and a second terminal. The converter includes moreover a half-bridge, i.e. a first and a second electronic switch, which is configured to selectively connect the first terminal of the primary winding to a supply signal or to ground, wherein a respective capacitance and a respective diode are associated to each switch of the half-bridge. Finally, the converter also includes, on the primary side, at least one capacitor which is connected between the second terminal of the primary winding and the supply signal and/or ground.
In various embodiments the converter includes, on the secondary side of the transformer, a ZETA converter. Specifically, in various embodiments, such a ZETA converter includes three branches which are connected in parallel, wherein:
a) the first branch includes a first capacitor, which is connected in series with the secondary winding of the transformer, and therefore the first branch includes the magnetization inductance of the transformer (and possible further inductors connected in parallel with the primary and/or the secondary winding of the transformer) and the first capacitor,
b) a second branch including a diode, and
c) a third branch including a second capacitor, connected in series with a second inductance, wherein the output is connected in parallel with the second capacitor.
In various embodiments, the switching of the half-bridge switches is driven by a control unit. For example, in various embodiments, the control unit drives the first and the second electronic switches with four time intervals which are repeated periodically:
a) a first time interval, wherein the first switch is closed and the second switch is open, so that a current flow from the supply signal flows through the primary winding of the transformer and such a current flow increases;
b) a subsequent second time interval, wherein the first switch is open and the second switch is open;
c) a subsequent third time interval, wherein the first switch is open and the second switch is closed; and
d) a subsequent fourth time interval, wherein the first switch is open and the second switch is open.
Specifically, in various embodiments, the capacitance associated to the first switch is charged and the capacitance associated to the second switch is discharged during the second interval, and vice versa the capacitance associated to the first switch is discharged and the capacitance associated to the second switch is charged during the fourth interval, which allows a switching of the switches at zero voltage.
Specifically, while in the paper by WU TSAI-FU the leakage inductance contributed to the discharging the capacitance associated to the first switch and to the charging the capacitance associated to the second switch during the fourth time interval, according to the present disclosure such a function is now performed via the inductances of the ZETA converter, i.e. the magnetization inductance of the transformer (and possible other inductances connected in parallel with the primary and/or the secondary windings of the transformer), and the inductance of the third branch of the ZETA converter are dimensioned in such a way that such inductances supply, during the fourth time interval, a current which charges the capacitance associated with the second switch and discharges the capacitance associated to the first switch.
Thanks to such a sizing, the equivalent capacitance at the intermediate point of the half-bridge including the capacitances associated to the switches of the half-bridge may also range between 200 pF and 1.5 nF.
As a matter of fact, by knowing such an equivalent capacitance, inductances may be dimensioned as a function of the input voltage, of the duty cycle of the converter, i.e. the duration of the first time interval with respect to the duration of the switching period, of the output current supplied through the output, and the equivalent capacitance of the intermediate point of the half bridge.
In various embodiments, during the third time interval an oscillation of a resonant circuit occurs which includes the capacitor(s) connected to the second terminal of the primary winding, the capacitor connected in series with the secondary winding of the transformer and the leakage inductance of the transformer. In various embodiments, such a resonant circuit is dimensioned so that:
a) during the third time interval there are present one or various complete half periods of the resonance of such a resonant circuit, and
b) the current flowing through the diode is zero at the end of the third time interval.
Therefore, thanks to such a sizing, also the diode of the ZETA converter may be switched at zero current.